1. Field of the Invention
The present invention relates to the field of handling the gas phosphine without generating dangers.
2. Brief Description of the Background of the Invention Including Prior Art
Phoshine (=hydrogenphosphide, formula PH3) is used since more than 50 years as pesticide and for pest-prophylaxis in the area of storage-protection, especially in the area of fodder and food.
Phosphine has some special properties which are of great importance for the modern pesticide-practice.
Thus, for example, phosphine is only very little soluble in water, oil and grease and will be nearly not absorbed by most of the food-materials. Phosphine is also not reacting with treated food with the consequence that the food carries no phosphine-residues after the treatment.
Due to the fact that phosphine is principally very little soluble and has a specific weight similar to air, phosphine can be distributed easily in big stores and storage containers.
When air will be put into contact with stores which have been treated by phosphine, the phosphine will be oxydized quickly to phosphorus-acid by UV-radiation with the consequence that no compounds will be created which contaminate the environment.
Further it is stated that in the last years it has been detected that phosphine is created spontaneously in the free nature.
On the other hand it has to be said that the above mentioned treatment with phosphine is accompanied by some problems. Phosphine present together with air results in a mixture which is easily flammable.
The limit of composition before ignition of this gas-mixture is 1.8% phosphine in air. If the concentration of phosphine surpasses this limit then the phosphine has an increased tendency of self-ignition.
On the other hand in the area of pesticides no high concentrations of phosphine are however necessary. An effective treatment will be achieved already by a phosphine-concentration of 200 to 300 ppm with the consequence that in the pesticide area there should not exist a danger of ignition. Nevertheless it cannot be excluded that under certain conditions sometimes higher phosphine-concentrations will arise with the consequence of self-ignition.
The experts in this field have presented a large number of proposals to solve this problem.
In order to use phosphine without danger it has been proposed for example to press paraffinated aluminiumphosphide (formula AlP) together with ammoniumcarbamate into tablets. These tablets will be exposed to the humidity of the air with the consequence that the AlP will be transformed to PH3 and aluminum semi hydroxide AlOOH slowly by hydrolysis. The originated phosphine will be thinned down instantly by the surrounding air and thereby preventing ignition. Unfortunately the solid residue of this method has a content of 1-3% of AlP which has not reacted. Furthermore also magnesiumphosphid Mg3P2-compounds have been prepared and used in the pesticide-area, which will react with the humidity of air nearly totally with the consequence that the residues will have a very little content of poison.
The hydrolysis cannot be interrupted anymore after beginning of the reaction in all solid forms which are presented and which react with air-humidity to build phosphine. Furthermore the tablets must be dispersed in such a manner that no agglomerations will take place.
In present days it will be tried to use nitrogen-gas or C02 as thinning medium for phosphine in the gas-treatment field against pests. Hereby, it is absolutely necessary to use phosphine-gas, which is very much thinned in order not to exceed the ignition limit concentration of 1.8%. The gas will be transported in pipe-lines to the gas-treatment-objects. Thus it is possible to stop the supply of gas at any desired time and to add air as required to the subject composition. The disadvantage of this method is the fact that a great number of steel-cylinders, which contain thinned phosphine, must be transported, with the consequence that such methods work only with difficulty and are troublesome. Furthermore a generator has been created by which, beginning with Mg3P2, a mixture of air and phosphine will be produced. Unfortunately, also in using this generator, it cannot be excluded that under certain circumstances the ignition limit concentration will be exceeded with the consequence of self-ignition.
Therefore the problem is pure phosphine coming directly from the pressure-container with air up to under the ignition-limit and to use the produced gas-mixture against pests. By this method the transportation-cost is reduced considerably, and therefore the gas treatment is much cheaper.
Unfortunately, during the known thinning processes ignition cannot be excluded; in a case where a burning of phosphine has already started in the air, an extinguishing of such a flame can become very difficult. No process or method is known in the prior art by which it is possible to mix phosphine directly with air without a danger of ignition.
1. Purposes of the Invention
Ait is an object of the present invention to furnish a method for mixing phosphine directly with air without a danger of ignition.
These and other objects and advantages of the present invention will become evident from the description which follows.
2. Brief Description of the Invention
The present invention is based on the observation that phosphine, which has impurity-contents of Di- and Polyphosphine, has a higher tendency of self-ignition compared with pure phosphine, which has been produced from Mg3P2.
The present invention solves the problem to create a method and an apparatus which carry out a direct mixing of phosphine and air without any danger of ignition.
Surprisingly it has been found that by a mixing of phosphine and air under water the danger a self-ignition is eliminated. Hereby any relation phosphine/air can be held: When the phosphine will be thinned up to under the ignition limit concentration, the mixture will not be inflamed by a burner above water. If a point-like heating and ignition of the gas-mixture is carried out at the mixing-point under water, then the mixture is burning at the mixing-point only as long as the ignition-originator is present. Immediately after removing of the ignition-originator the flame will be disconnected by the water and extinguished.
It is possible acccording to this method to mix phosphine directly with air in the presence of water.
According to this invention, ordinary water and also water which contains anti-freezing-agents can be used in this device. Also thinned inorganic acids and thinned alkaline solutions or inorganic aqueous-solutions can be used.
This device is constructed in such a way that a quick thinning of the phosphine with air is effected under water with the consequence that the phosphine-air-mixture, which is leaving the water, is of a homogeneous nature. The most simple embodiment of this device, according to the invention, is consisting of a vertical pipe arranged to end under water, wherein the air is added through the pipe from below. This pipe has little openings in its wall through which the phosphine is flowing in, with the consequence that the mixture is always below the ignition limit concentration (=ignition limit).
The same effect can be achieved by using a water-jet-vacuum-pump whereby the phosphine is introduced into the water-jet and the air will be sucked in by the water-jet. A preferred embodiment of the device according to this invention can produce 100 m3 phosphine thinned with air per hour.
Thereby the gas-mixture can contain up to 1,8% phosphine without any problems. In order to be able to mix phosphine with air without dangerous side-effects, it is advisable to flush all pipe-lines, in which phosphine will be transported, with CO2, nitrogen or any other inert gas, before the beginning the process of phosphine generation. Then a fan or blower, which is transporting the air, will start to operate and come into function. It is further proposed to survey the air-stream in order to control any failure of air supply, since the air is necessary for thinning.
As soon as enough air and also enough water is present, the phosphine can be added. In case of any failure, like a blockage of the air-pipeline, a failure of the electric current or a surplus of phosphine, the addition of phosphine will be stopped automatically and all pipe lines will be flushed by an inert gas.
By this process, mixtures can be produced up to below the ignition limit. For normal gas treatments, such a high phosphine-content is not necessary, because much lower concentrations are sufficient. For the normal applications of the phosphine-air-mixture a phosphine-cocentration of 0,18% is used. In order to interrupt the working of the device, according to this invention, first the supply of phosphine will be stopped; then the pipe lines will be flushed by inert gas; then the fan will be stopped.
The novel features which are considered as characteristic for the invention are set forth in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.